The present invention relates to a nuclear magnetic resonance imaging apparatus to measure a nuclear magnetic resonance (hereinafter abbreviated as "NMR") signal from hydrogen, phosphorus and the like in a human body and to image the density distribution of nuclei, distribution of relaxation time, and the like.
In the prior art, an X-ray CT and an ultrasonic imaging apparatus are widely used as an apparatus for the nondestructive inspection of an internal structure such as of a human brain and abdomen. In recent years, a trial of performing a similar inspection using an NMR phenomenon has been attempted whereby many sorts of information can be obtained which are not obtained by an X-ray CT or an ultrasonic imaging apparatus.
In an imaging apparatus using an NMR phenomenon, a signal from an inspection substance must be separated and discriminated corresponding to the position. As one such method, intensity of a static magnetic field is provided with inclination along an arbitrary direction thereby the magnetic field intensity in each position is made different hence a resonant frequency or a phase encoding value in each position is made different so as to obtain information of the position. The basic principle of this method is reported in "Journal of Magnetic Resonance", vol. 18, pp 69 -83 (1975), "Physics in Medicine & Biology", vol. 25, pp 751-756 (1980) or the like.
Since SN (signal-to-noise) ratio in the NMR increases in proportion to the power of 1-1.5 of the static magnetic field H, the magnetic field intensity is being made as high as possible so as to improve the SN ratio. A transmitting and receiving probe (hereinafter referred to simply as "probe") which has been used until now is a solenoid or a saddle shape probe, and since its resonant frequency increases as the magnetic field intensity increases, the self resonant frequency of the probe and the NMR frequency may come close to each other or relation of both may be reversed. As a result, a problem of lowering of sensitivity in the receiving state or lowering of the generating efficiency of the high-frequency magnetic field in the transmitting state will be produced.
On the contrary, a probe of new type by Alderman et al. (called "the Alderman type probe") has been proposed. The basic principle of this probe is described in "Journal of Magnetic Resonance", vol. 36, pp 447-451 (1979).
In the imaging as above described, improvement of the efficiency of the probe for generating or receiving the high-frequency magnetic field becomes the important problem leading to improvement of the picture quality and decrease of the image pick-up time. One method of constituting a quadrature-phase detection probe (hereinafter abbreviated as a "QD probe") has been proposed by D. I. Holt et al. This probe theoretically improves the SN ratio .sqroot.2 times in comparison to the Alderman type probe. The basic principle of this method is described in "Journal of Magnetic Resonance", vol. 69, pp 236-242 (1986).
FIGS. 28, 29 and 30 show the QD probe, and FIG. 28 is a bird's eye view of the probe, FIGS. 29(a)(b)(c) are sectional views taken in lines A'--A', B'--B', C'--C' of FIG. 28, and FIG. 30 is a sectional view in the xz plane of FIG. 28, respectively. The QD probe is provided with a first arm pair comprising two conductor arms 3a and 3c in direction coincident with the direction (z-axis direction) of the static magnetic field and a second arm pair comprising two conductor arms 3b and 3d also in the z-axis direction, and wings 4a-4d extended laterally from each one end of the respective conductor arms are connected by capacitors 24a-24d and wings 4e-4h extended laterally from other end of the respective conductor arms are connected by capacitors 24e-24h. Further guard rings 1a, 1b are arranged in the vicinity of two sets of the wings respectively. The QD probe is combination of a first probe circuit having a port A as a feeding point and the first pair of arms as a current path and a second probe circuit having a port B as a feeding point and the second pair of arms as a current path, and a matching circuit composed of capacitors 24q, 24r shown in FIG. 31 is connected to the ports A, B respectively. The matching circuit is further connected through a cable ring shown in FIG. 32 to a transmitter and a receiver of the NMR apparatus, thus transmitting signals of high frequency are supplied to both ports in the phase difference of 90 degrees, and receiving signals of high frequency from both ports having the phase difference of 90 degrees are combined in the same phase and guided to the receiver.